The objective of this proposed project is to develop and utilize a reliable method(s) for the morphometric assessment of physiologic events (constriction, dilation) in cerebral arteries and arterioles, especially the smaller, penetrating cortical vessels. To our knowledge, a reliable method of capturing (fixing) vessels in their in vivo caliber does not exist. Two experimental approaches will be applied. The first entails perfusion fixation of rat brains following photographic documentation of pial vessel diameters. Perfusion will be undertaken at pressures equal to the animal's blood pressure and variables of the perfusate will include type of aldehyde, temperature, concentration, osmolality and buffer. Following perfusion fixation, vessels (pial and cortical) will be cross-sectioned, processed, embedded, stained and in vitro vessel diameters will be compared with in vivo diameters pre-and post perfusion in order to determine the most effective and reliable method of preserving vessel caliber by perfusion. The second approach entails rapid freeze substitution. Preliminary work suggests that this method of fixation may prove extremely reliable as a means of rapidly fixing and capturing vessles at the precise caliber they possessed in vivo immediately prior to fixation. Methods of fixation derived from these studies will be applied to comparison of pial and parenchymal brain vessels in terms of their relative contributions as resistance vessels and also to the reactions of these vessels in the hypertensive state.